Printers typically employ stored calibration data that maps input image signals into the device's output color space, e.g., to reduce non-linearities in the device's color response. This calibration profile is usually set at the factory, and not thereafter altered.
For many applications, such arrangements are satisfactory. However, in more demanding print environments—such as high end graphic arts work—a printer may be recalibrated periodically. Such recalibration can correct for changes in printer operation due to factors such as changes in ambient temperature, non-uniformities of consumables (e.g., inks), and differences in printing substrates.
U.S. Pat. No. 6,075,888 describes one technique for recalibrating the stored color profile of a printer. Data corresponding to a series of input test colors are provided to the printer, and are mapped to the device's output color space using the stored color profile data. Resulting color patches are printed. Colorimetric values of these patches are then measured, and the results are compared with the input test colors. Differences identified in this comparison are used to adjust the printer's stored color profile, so as to bring the calorimetric measurements of the printed output and the input test colors into better agreement.
While such a recalibration procedure may be practical in some settings, it is impractical in others. Among its disadvantages, the foregoing technique requires expensive colorimetric measuring equipment, and considerable technical expertise. Moreover, it is a prolonged procedure, ill-suited for environments in which regular recalibration may be desirable.
One setting in which the above-detailed procedure is unsuitable is in connection with printers used to produce digitally watermarked photo ID cards, such as driver's licenses. For optimal results, it is desirable to recalibrate such printers periodically (e.g., when the printer ribbon is changed), so that the print quality of the resulting ID card is uniformly excellent, and the watermark information is well concealed yet reliably readable. However, the operator of such a photo ID printing system is typically a person who is relatively unskilled in printer technology and colorimetry, and who lacks the time or equipment to engage in a prolonged procedure.
Accordingly, there is a need for a printer re-calibration procedure that can be performed quickly, without expensive equipment, and without a high level of operator expertise.
In accordance with one embodiment, an operator performs field recalibration of a printer by printing a test graphic on a sample of the target substrate, viewing the printed graphic to discern the visibility of one or more contrasting features, and indicating (e.g., using a computer user interface) whether or not such features are visible. Based on the operator's reports of visibility, it can be determined whether the correct stored calibration data has been used, and whether it has been tweaked correctly. If not, appropriate adjustments can be made.
In some embodiments, the operator prints and assesses three test graphics, respectively evidencing the printer's ability to accurately reproduce image highlights, mid-tones, and shadows. The operator's feedback is used to adjust the stored calibration data so as to better linearize the printer's response across the three ranges.
In other embodiments, the observations can be made by a sensor disposed within the printer housing.
In accordance with a more general embodiment, a printer is instructed to print a test graphic comprised of elements that differ slightly in tone value. By reference to a difference (or absence thereof) between two or more of the elements as actually printed, a corresponding change can be made to the stored printer calibration data.
In one embodiment, the difference is the presence or absence of visible contrast between two features in the printed test graphic. In another embodiment, there are two visible contrast changes in the test graphic, and the distance therebetween is used in determining a change to the stored printer data.
Another aspect of the invention is a printing system with stored profile data, and an internal sensor system by which the foregoing difference(s) in actual printed output can be assessed. Still another embodiment is a printing system that includes a user interface through which operator assessments of printer performance can be received and used in adjusting the stored profile data.
One particular embodiment employs a two-step calibration procedure:
(a) normalize the printer and bring it back to a known condition; and
(b) apply dynamic range adjustment, and check if correct table is applied.
In this embodiment, the software imaging application used with the printer, or the printer driver, is caused to print a gray balance test target that includes several differently-composed (e.g., by different R, G, B values) grey patches. An operator (or a built-in electronic color sensor) makes a visual comparison to identify the patch that most closely matches the color gray on a reference guide shipped with the printer. The selected patch is identified to a software program. This program causes the printer to return to a known condition, tweaking the dynamic range adjustment table and thereafter producing correctly color calibrated images. To confirm correct calibration, a further test pattern can be printed and again visually inspected.
The foregoing and other features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.